1. Field of the Invention
The present invention relates to mixtures of disulfonated anthraquinones and more specifically to mixtures of disulfonated anthraquinones having high solubility and utility in the Stretford Process, which is defined below.
2. Description of the Prior Art
Anthraquinonedisulfonic acids in addition to their use as precursors in the dye industry, are employed in the well-known Stretford Process. The Stretford Process is a liquid purification process for the removal of hydrogen sulfide from gas streams and liquid hydrocarbons. In the Stretford Process a hydrogen sulfide containing fluid such as a stream of pretroleum or coal gas is intimately contacted with an aqueous alkaline solution of salts of one or several anthraquinonedisulfonic acids and of vanadium salts, e.g., sodium meta vanadate. The hydrogen sulfide is oxidized, liberating elemental sulfur which separates and is filtered off. The anthraquinonedisulfonic acids function as hydrogen acceptors which are capable of being easily regenerated for reuse without significant loss. The regeneration is carried out in a separate operation by oxidation with air.
Many water-soluble anthraquinonedisulfonic acids will function as hydrogen acceptors in the aqueous system of the Stretford Process. For practical reasons, however, anthraquinonedisulfonic acids having high solubility in the process and which are not sensitive to the "salting-out" effect of the inorganic cations present in the process are preferred since smaller absorbtion units can, by using more concentrated solutions, provide superior and more economical performance. It is also preferred that the anthraquinonedisulfonic acids react rapidly in the acceptor step and the regeneration step.
Certain anthraquinonedisulfonic acids have been found to perform well in the Stretford Process. Of especial value are 2,7-anthraquinonedisulfonic acid, 1,6-anthraquinonedisulfonic acid and 1,7-anthraquinonedisulfonic acid.
The methods previously known by which these anthraquinonedisulfonic acids are produced, however, suffer serious disadvantages. If anthraquinone is disulfonated in the absence of mercury or its ions, a mixture of 2,6- and 2,7-anthraquinonedisulfonic acid is obtained. The 2,7-isomer is reasonably useful as a Stretford desulfurizing material, being sufficiently water soluble and being capable of fairly fast regeneration. The 2,6-isomer, however, is relatively insoluble in the system normally used in the Stretford Process, and only comparatively small amounts can be tolerated in units of efficient design. Since the isomers are formed in relatively equal amounts when anthraquinone is disulfonated, unless a use exits for the the 2,6-isomer, the synthesis and separation of the 2,7-isomer is not economically attractive. Thus, the ready availability of the 2,7-isomer is directly dependent on the commercial demand for the 2,6-isomer and the price of the 2,7- becomes unacceptably high if the market for the 2,6- is poor.
Other methods suitable for producing anthraquinonedisulfonic acids having utility in the Stretford Process require the use of mercury or certain of its salts. Procedures based on such technology, however, must have a mercury-removing step for the products and the effluent from the processes will contain intolerable amounts of derivatives of this dangerous element. The techniques by which mercury can be removed are not well developed and efficient, and would not be economical even if they were available due to the high capital investment required to build the large and complicated equipment.